Nikon Warranty: "Date of Purchase" definition?

When the warranty says you must keep your receipt to claim warranty service, all I have from Amazon is the Invoice and on the Invoice it doesn't put the camera serial number or anything like that, just the name so how would Nikon knows which camera belong to what invoice. Will that be good enough for Nikon when it time for me to do warranty service? Because when I buy from Best Buy etc, the serial number will be on the receipt so everything is certain.